1. Field of the Invention
The present invention relates to a yttrium silicate based phosphor and a method for synthesizing a yttrium silicate based phosphor by surface coating.
2. Description of the Related Art
Field emission devices., which operates with a driving voltage as low as 1 kV or less, requires a high emission phosphor. When an existing sulfide based phosphor is deposited in a FED, the chemical stability of phosphor is lowered and charges accumulate at the phosphor surface during operation. To account for these problems, many approaches have been conducted into oxide based phosphors. As a result, yttrium silicate based phosphors have been considered to be a superior oxide series phosphor.
Yttrium silicate based phosphors are thermally and chemically stable compared to sulfide based phosphors. However, the chromaticity coordinates and luminance of the yttrium silicate based phosphors are inferior. An existing green-light-emitting sulfide based phosphors for use FEDs, for example, ZnS:Cu, Al phosphors, have chromaticity coordinates (x=0.333, y=9,614). On the other hand, oxide based phosphors, such as Y2SiO5:Tb phosphors, have chromaticity coordinates (x=0.333, y=0.582). In particular, the luminance of the oxide based phosphor is about 30 to 40% of that of sulfide based phosphors. Sulfide based phosphors, which are in use, are effective for FEDs operated with a high driving voltage (5 to 10 kV). However, when a driving voltage is as low as 1 kV or less, such sulfide based phosphors have the problems of low luminance and charge build-up, as previously described. Thus, there is a need for a new oxide based phosphor having effective emission at low voltages.
To solve the above problems, it is an objective of the sent invention to provide a yttrium silicate based phosphor with enhanced luminance, chromaticity coordinates and aging characteristics, and a method of synthesizing the same by surface coating, in which a thermally and chemically stable yttrium silicate based phosphor is coated with a strontium (Sr)-thiogallate phosphor having superior luminance and chromaticity coordinates.
According to an aspect of the present invention, there is provided a yttrium silicate based phosphor synthesized by surface coating a yttrium silicate phosphor, such as Y2SiO5:Tb phosphor, coated with a strontium (Sr)-thiogallate phosphor, having the formula Y2SiO5:Ln+SrGa2S4:Lm, where Ln is a lanthanide element selected from the group consisting of gadolinium (Gd), terbium (Tb), cerium (Ce) and europium (Eu), and Lm is an element selected from the group consisting of Eu, Tb and Ce.
According to another aspect of the present invention, there is provided a method for synthesizing a yttrium silicate based phosphor by surface coating, comprising the steps: (a) mixing Ga(NO3)3, Lm(NO3)3, where Lm is Eu, Th or Ce, and SrCO3, and stirring the mixture in a solvent until the SrCO3 is completely dissolved; (b) after SrCO3 is completely dissolved in step (a), adding a Y2SiO5:Ln phosphor, where Ln is Gd, Eu, Tb or Ce, to the mixture from step (a); (c) adding a sodium hydroxide (NaOH) to the mixture containing the Y2SiO5:Ln to adjust pH, and adding sulfur (S) to the pH-adjusted mixture; (d) drying and sieving the sulfur containing mixture from step (c); and (e) firing the sieved phosphor mixture from step (d) at 750xc2x0 C. for 4 hours.
Preferably, the resulting mixture obtained through steps (a) through (e) is a yttrium silicate based phosphor having the formula Y2SiO5:Ln+SrGa2S4:Lm, where Ln is a lanthanide element selected from the group consisting of gadolinium (Gd), terbium (Tb), cerium (Ce) and europium (Eu), and Lm is an element selected from the group consisting of Eu, Tb and Ce.